A hard-disk drive (HDD) is a non-volatile storage device, which is housed in a protective enclosure, that stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the space between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. To provide a uniform distance between a read/write head and the surface of a magnetic-recording disk, an actuator relies on air pressure inside the hard drive enclosure to support the read/write heads at the proper distance away from the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. That is, the air pulled along by a spinning magnetic-recording disk forces the head away from the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away.
A write-head of a HDD records data onto the surface of a magnetic-recording disk in a series of concentric tracks. When a write-head writes data to a desired track of a magnetic-recording disk, it is important for the write-head to be located close to the desired track; failure to do so may result in a squeeze event, which may compromise data integrity and throughput, and in extreme cases, may result in hard errors and data loss. A squeeze event occurs when a write-head writes data too close to or overlapping with an adjacent track such that there is not enough of the adjacent track left for the adjacent track to be read properly by a read-head.
References markers may be recorded in each track of a magnetic-recording disk. These reference markers are referred to as servo information. To help properly position the write-head when writing data, a HDD employs a servo mechanical control loop to maintain the write head in the correct position using the servo information stored on the magnetic-recording disk. When a read-head reads the servo information (servo information being read may be referred to as a position-error signal, or PES), a relative position of the head may be determined by a servo processor to enable the position of the head, relative to the desired track, to be adjusted if necessary.
In the servo-mechanical control loop employed by a typical hard-disk drive, the position of the read/write head is usually known to a high degree of precision; however, the relatively slow response time in moving the mechanical parts of a HDD, such as the head-arm assembly (HAA) on which the read/write head is affixed, makes it difficult to accurately and rapidly correct errors in the position of the read/write head.
The possibility of a squeeze event increases with an increase in the density of tracks on the magnetic-recording disk. As a result, writing data in a shingle writing process is particularly susceptible to a squeeze event, as shingle writing will typically only be used in systems having extremely high track densities. In shingle writing, each data track is partially overwritten when an immediately contiguous data track is written.